Cement plugs are placed in wellbores for a variety of reasons; for curing wellbore instability or losses, plugging a wellbore or a portion of it, abandoning a wellbore or a section of it, providing a base for initiating a deviation or kick-off and more. Cement plugs are constructed by pumping a relatively small amount of cement slurry down a drill pipe where it later sets solid.
FIG. 1 shows a section of a well in which a cement plug is being set. The well 10 has a viscous pill 12 set in the well 10 at the bottom of the desired plug location. The end of a drill pipe 14 is then positioned just above the pill 12 and cement 16 is pumped down the drill pipe 14 into the well to form the plug 18 on the pill 12. As the plug builds up, the drill pipe is pulled back so that it does not remain in the plug 18 when it sets. Once all of the cement has been pumped into the well 10 and the plug 18 has formed, the drill pipe 14 can be withdrawn and other operations continued.
Drilling operations resume once the cement plug has set and developed enough strength to fulfil its objective, for example to initiate side tracking or to plug conductive fractures. There is great uncertainty however about when it is safe to resume drilling operations, to mitigate the risk of resuming drilling operations too soon, it is common practice to wait extra time, typically 24 hours.
Achieving proper placement of the cement plug presents a challenge as does uncertainty in the downhole temperature; frequently too much time is allowed for the cement plug to become set before an attempt to continue well operations is made.
A further problem is the risk of contamination, which leads to the cement not setting in the desired period of time and not achieving the required strength. The cement slurry may initially become contaminated whilst being pumped into the drill pipe and later when being jetted in the underlying drilling fluid. Some contamination also occurs in the drill pipe or casing annulus as the displacement of the drilling fluid is seldom perfect as the drill pipe is not centralized and some mud is trapped on the low or narrow side.
Another problem is cement plug slumping into the lower part of the well as it is usually denser than the drilling fluid. Frequently a viscous pill is placed to combat this. However frequently the plug does not have sufficient viscosity when placed and therefore does not provide support for the cement allowing the cement to slump to the bottom of the well leading to a failed plug.
This invention seeks to overcome many of the challenges highlighted above by providing real-time measurements of wellbore properties during cement plug placement.